Carbonatites

rocks of magmatic or metasomatic origin, composed primarily of carbonates (calcite, dolomite, ankerite) and spatially related to complexes of ultrabasic-alkalic composition. The term “carbonatite” was introduced by the Norwegian petrographer W. Brûgger (1921). He also proposed that calcite carbonatites be called sövites, dolomite carbonatites be called rauhaugites, biotite-dolomite veined carbonatites be called befor-sites, and red-colored carbonatites (in which the carbonate is partially replaced by iron oxides, primarily hematite) be called redbergites.

The complexes of ultrabasic-alkalic rock in which carbonatites are found are generally located along major faults on the platforms. They may be “blind,” that is, not reach the earth’s surface, or “open,” reaching the surface in the form of volcanoes that erupt carbonatite lava (the Oldoinyo Lengai volcano in Tanzania). Geophysical data show that the complexes are dozens of kilometers deep. Carbonatites make up the central parts of the complexes, forming stocks and chimney deposits measuring from 0.1 to 15–20 km and more in area as well as irregularly shaped deposits, branching zones, stockworks, and circular, conical, and radial dikes. In open-type complexes they fill volcanic vents, frequently cementing brecciated volcanic rocks. Where carbonatites develop on ultrabasites and ijolites, in some complexes there occur forsterite-apatite-magnetite rocks with small amounts of calcite (phoscorites, kamaphorites); these rocks are sometimes high-quality magnetite ores (for example, Kovdor on the Kola Peninsula in the USSR) or are rich in apatite (the Palabora Massif in the Republic of South Africa). When carbonatites develop on nepheline syenites, aureoles of albitites with tantalum-niobium mineralization often form.

Carbonatites are multistage formations that form in the temperature interval between 600° and 300°C. Early-stage carbonatites consist of calcite, diopside or forsterite, biotite or phlogopite, apatite, and magnetite. They are enriched with Ti, Zr, Ta, Nb, and U.

Carbonatites of the late stages consist of 80–95 percent dolomite or ankerite and calcite, and, more rarely, siderite and stron-tianite. They contain alkaline amphiboles, serpentine, ferroferriphlogopite, aegirite, chlorite, and epidote. Sulfides typically appear (pyrite, pyrrhotite), as well as fluorite, barite, magnetite, rutile, pyrochlore, lueshite, columbite, fersmite, bur-bankite, bastnaesite, parisite, carbocernaite, and ancylite. Carbonatites are characterized by high concentrations of Sr, Ba, F, Nb, Ce, Th, Pb, Zn, and Mo.

Carbonatites and their associated rocks are an important form of mineral deposit. Related to them are large deposits of phlogopite and vermiculite (Kovdor and Gulinskoe in the USSR), iron (Kovdor in the USSR, Palabora in the Republic of South Africa), phosphorus (Palabora in the Republic of South Africa, Sukulu in Uganda), and rich deposits of niobium ore (Araxá in Brazil, Lueshe in Zaïre, Oka in Canada), well as deposits of tantalum (Nqombwa, Zambia), zirconium (Palabora, Republic of South Africa), rare earths (Mrima, Kenya), copper (Palabora, Republic of South Africa), fluorite (Tagna, USSR), and raw materials for cement and lime (Tororo and Sukulu in Uganda). In addition, it is possible to extract barite and strontianite from some of the deposits. Under conditions of hypergenesis a weathering mantle develops on carbonatites; the content of useful components (apatite, pyrochlore, bastnaesite) in it is three to five times greater than in bedrock.